1. Field of the Invention
The present invention relates to a zone melting apparatus and particularly to a zone melting apparatus for monocrystallizing a thin semiconductor layer on an insulator layer.
2. Description of the Prior Art
Referring to FIG. 1, an example of a conventional zone melting apparatus is schematically illustrated (see "Zone-melting recrystallization of encapsulated silicon films on SiO.sub.2 -morphology and crystallography", Appl. Phys. Lett., 40(1982)158, by M. W. Geis et al.). A lower heater 11 such as a carbon plate supports a wafer 13 to be described afterwards. An upper elongated linear heater 12 moves slowly above the wafer 13 in the direction of the arrow 10.
FIG. 2 shows a layered structure of the wafer 13 including a thin polycrystalline or amorphous silicon layer to be monocrystallized. On an upper major surface of a monocrystalline silicon substrate 15, a silicon dioxide layer 16 of approximately 0.5 .mu.m in thickness is formed and on this layer 16, a polycrystalline silicon layer 17 of approximately 0.5 .mu.m in thickness to be monocrystallized by zone melting is formed. Further on the polycrystalline silicon layer 17, protection layers comprising a silicon dioxide layer 18 of approximately 2 .mu.m and a silicon nitride layer 19 of approximately 30 nm is formed. The protection layers serve to protect the silicon layer 17 from the environment and to reduce the influence of the surface tension of the silicon layer 17 subjected to the zone melting.
In the operation for monocrystallizing the polycrystalline silicon layer 17, the wafer 13 is heated to 1200.degree. C. by the lower heater 11. The upper elongated heater 12 at 2000.degree. C. located at a height of approximately 1mm to 2 mm from the top surface of the wafer moves from one end of the wafer 13 in the direction shown by the arrow 10 at a rate of, for example, 2 mm/sec, so as to sweep the whole area of the wafer 13. During the sweeping, the silicon layer 17 is melted in a linear zone 14 directly below the elongated heater 12. The melting zone 14 moves throughout the silicon layer 17 according to the movement of the upper elongated heater 12, whereby the silicon layer 17 in the wafer 13 is monocrystallized. This monocrystallizing operation is performed in an atmosphere of inert gas such as argon.
FIG. 3 is a schematic illustration showing another conventional zone melting apparatus. A wafer 30 as shown in FIG. 2 is provided on a wafer support plate 36. A zone melting tubular lamp 22 with a convergent mirror 220 is provided above and parallel to the wafer 30. The convergent mirror 220 focuses the light from the tubular lamp 22 linearly onto the polycrystalline silicon layer 17 in the wafer 30 so that a linear melting zone is formed in the silicon layer 17.
Wafer heating tubular lamps 21a, 21b, 21c, 21d and 21e are provided in a plane below and parallel to the wafer support plate 36 and have reflection mirrors 210a, 210b, 210c, 210d and 210e, respectively. These wafer heating tubular lamps 21a to 21b are disposed with equal spacings between them and parallel to the zone melting tubular lamp 22. The reflection mirrors 210a to 210e reflect respectively the light from the wafer heating lamps 21a to 21e toward the support plate 36 so as to prevent scattering loss of the light. The light from the wafer heating lamps 21a to 21e irradiates and heats the whole surface of the support plate 36 so that the whole area of the wafer 30 is heated. In such an apparatus as described above, a relatively large number of wafer heating tubular lamps are provided so as to heat the wafer 30 uniformly.
In the operation, the wafer 30 along with the wafer heating tubular lamps 21a to 21e moves relatively to the zone melting tubular lamp 22 in the direction shown by the arrow 120. The arrow 120 is normal to the axis of each of the wafer heating tubular lamps 21a to 21e. During the movement of the wafer 30, the light from the zone melting tubular lamp 22 irradiates linearly the silicon layer 17 in the wafer 30 so as to sweep the whole area of the wafer 30. During the sweeping, the linear melting zone moves in the silicon layer 17 from an edge to the opposite edge thereof between the upper and lower silicon dioxide layers 18 and 16, whereby the silicon layer 17 is monocrystallized.
In a practical application of zone melting by such a conventional apparatus as described above, since the wafer is heated from both of the upper and lower sides, heat cannot easily escape from the central portion of the wafer and therefore, the silicon layer 17 in the wafer having a large area of more than 4 inches in diameter cannot be monocrystallized by one pass of zone melting. In other words, in such a conventional apparatus, it is difficult to obtain a monocrystalline layer that is homogeneous over the whole area because it sometimes happens that even if the central portion of the silicon layer 17 is melted, the edge portions thereof are not melted or, if the edge portions of the silicon layer 17 are melted, the central portion thereof is heated excessively.